The present invention relates generally to a gas turbine engine variable stator vane torque tube assembly and, more particularly, to a torque tube assembly having improved bearing life.
Variable pitch stator vanes are commonly used in compressors of gas turbine engines. The variable pitch stator vanes or, more simply, variable stator vanes are pivotably mounted in axially spaced rows inside a compressor case of the engine. Each of the vanes pivots on a spindle having an axis extending radially with respect to an engine centerline to vary the pitch of the vane. Levers on the outboard ends of the spindles simultaneously pivot each of the vanes in a row. The levers form part of a bell crank mechanism mounted outside the compressor case. Some engines include a torque tube assembly operatively connecting the bell crank mechanism to a variable stator vane actuator mounted outside a fan duct surrounding the compressor case. The torque tube assembly includes an elongate torque tube extending through an access hole in the fan duct from the bell crank mechanism to the actuator.
During operation, the elongate torque tube rotates in the access hole about a longitudinal axis of the tube extending generally radially relative to the engine centerline from the bell crank mechanism to the actuator. Because the compressor case and the fan duct operate at significantly different temperatures, the relative axial positions of the compressor case and the fan duct change during engine operation. As a result, the outboard end of the torque tube tilts rearward as the engine heats up during engine operation. Thus, the torque tube must be free to tilt and rotate in the access hole. To facilitate this motion, a spherical bearing assembly is positioned between the torque tube and the access hole. The bearing assembly includes a spherical bearing mounted on the torque tube and a housing adapted for receiving the bearing mounted over the access hole on the fan duct.
Some conventional variable stator vane torque tube assemblies have housings tilted to match the calculated torque tube tilt angle during engine operation. FIG. 1 illustrates a cross section of a bearing assembly used in one such conventional variable stator vane torque tube assembly. This bearing assembly includes a housing tilted at an angle of less than about four degrees. However, in some instances torque tubes tilt rearward by as much as thirteen degrees or more during steady state engine operation. The resulting asymmetric loading developed in the bearing assembly during engine operation when the torque tube is disposed at an angle with respect to the housing has sometimes resulted in premature bearing assembly failure. Accordingly, there is a need for a torque tube assembly with a bearing that performs more reliably and has improved life under such operating conditions.
Briefly, the present invention includes a bearing assembly for use in a gas turbine engine to rotatably mount a torque tube in an access hole extending through a fan duct of the engine. The torque tube connects a variable stator vane bell crank positioned inside the fan duct to an actuator positioned outside the fan duct. The bearing assembly comprises a bearing adapted for mounting on the torque tube having a spherical external engagement surface. Further, the assembly comprises a housing having a flange adapted for mounting on the fan duct adjacent the access hole and an annular interior surface having an inner edge and an outer edge. The interior surface is sized and shaped for rotatably receiving the spherical external engagement surface of the bearing so the spherical engagement surface and the annular interior surface make contact along an annular contact area centered on a contact plane lying between the inner edge and the outer edge of the interior surface. The contact plane is tilted with respect to the flange at an angle greater than five degrees.
In another aspect, the invention includes a torque tube assembly for use in a gas turbine engine to connect a variable stator vane bell crank positioned inside a fan duct of the engine to a variable stator vane actuator positioned outside the fan duct. The torque tube assembly comprises an elongate torque tube having an inner end adapted for connection to the variable stator vane bell crank, an outer end adapted for connection to the variable stator vane actuator, and a longitudinal axis extending between the inner end and the outer end. Further, the assembly comprises a bearing mounted on the torque tube having a spherical external engagement surface and a housing having a flange adapted for mounting to the fan duct and an annular interior surface having an inner edge and an outer edge. The interior surface is sized and shaped for rotatably receiving the spherical external engagement surface of the bearing so that when the spherical engagement surface and the annular interior surface make contact along an annular contact area centered on a contact plane extending parallel to the outer edge of the interior surface of the housing, the torque tube axis extends at an angle of less than 85xc2x0 from the flange.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.